1. Field of the Invention
The present invention relates to an electrifier for charging to a predetermined polarity or potential a charge target member (image bearing member), such as an electrophotographic photosensitive member or an electrostatic recording dielectric member, and to an image forming apparatus equipped with this electrifier, such as a copier or a printer.
2. Related Background Art
A contact electrification means using charge accelerating particles is disclosed, for example, in Japanese Patent Application Laid-Open Nos. 10-307454 to 10-307459. In this contact electrification means, charge accelerating particles are introduced at a charge nip, which comprises the contact portions of a charge target member and a contact charge member (hereinafter referred to as a charge roller) that is generally provided as a roller member and that contacts the target charge member, so that an injection charging mechanism dominates a discharge charging mechanism.
The charge accelerating particles are conductive particles used as auxiliary means for charging. The charge accelerating particles can be arbitrary conductive particles, including particles of a metal oxide such as conductive oxide zinc, or particles of other conductive inorganic materials or organic material mixtures, having a diameter of 0.1 to 5 xcexcm, for example, and a volume resistivity equal to or lower than 1xc3x971012 xcexa9xc2x7cm, or more preferably, equal to or lower than 1xc3x971010 xcexa9xc2x7cm.
Because of the presence of the charge accelerating particles at the charge nip, the speed of the charge roller constituting the contact charge member can differ from that of the target charge member it contacts. At the same time, since the charge roller closely contacts the target charge member via the charge accelerating particles, charge failures due to insufficient contact seldom occur. That is, since there is no intervening gap separating the charge accelerating particles from the target charge member at the nip, a preferable charging property is obtained by rubbing the charge accelerating particles that are present against the surface of the target charge member, thereby directly injecting an electric charge into the target charge member (injection charging). In other words, because of the presence of the charge accelerating particles, the dominant process performed by the injection and charging mechanism is the electrification of the target charge member using the charge roller.
Therefore, electrification efficiency is increased, an objective that can not be achieved with the conventional roller charging, a voltage substantially as high as the voltage carried by the charge roller, which is the contact charge member, can be applied to the target charge member, and only a simple structure is required to implement ozonless injection charging at a low applied voltage. Injection charging is effective as the charging means for an electrophotographic image forming apparatus or an electrostatic image forming apparatus, and provides a uniform charge having a predetermined polarity and potential for an image bearing member, such as an electrophotographic photosensitive member or an electrostatic recording dielectric member.
Further, for injection charging for which the charge accelerating particles are used, a cleaner-less image forming apparatus can uniformly electrify an image bearing member (hereinafter referred to as a photosensitive member) constituted by a target charge member. As part of a specific configuration, the charge accelerating particles are mixed with a developer (hereinafter referred to as toner), and at a developing portion, this mixture is supplied by a developing device to the surface of the photosensitive member. At a transfer portion, primarily only the toner is transferred to a transferring material, while the charge accelerating particles are supplied to a charge nip at the portion whereat the contact charge member contacts the photosensitive member. As a result, in a cleaner-less image forming apparatus, ejection charging makes it possible to uniformly electrify a photosensitive member. A configuration whereby charge accelerating particles are supplied by developing means to the charge nip at the portion whereat the contact charge member contacts the photosensitive member is disclosed in Japanese Patent Application Laid-Open No. 10-307455, for example.
As the photosensitive member is sequentially rotated, following the transfer process residual toner reaches the developing portion via the electrification means, and is removed (collected) from the surface of the photosensitive member by the developing device at the same time as the developing is performed (toner recycle process).
A well known cleaning process that is performed simultaneously with the development of an image is one where, at the succeeding development process during the image forming, i.e., during the process for continuously electrifying and exposing the photosensitive member to form and to develop a latent image, toner remaining on the photosensitive drum following the transfer process is collected by employing a fogging removal bias, i.e., a fogging removal potential difference Vback constituting a potential difference between a direct-current voltage applied to the developing device and the surface potential of the photosensitive member (Japanese Patent Application Laid-Open No. 10-307456).
Since the charge accelerating particles intervene at the charge nip, the abutting pressure applied between the charge roller and the photosensitive member can be increased, while the contact torque of the charge roller is reduced and the residual transfer developer does not pass through the charge nip between the charge roller and the photosensitive member.
In the above described system, personified by a transfer type cleaner-less image forming apparatus that employs, as means for electrifying an image bearing member, contact charge means that uses the charge accelerating particles, where a residual transfer developer is carried to the contact charge means, not only must the image bearing member be electrified, appropriate electric charges must also be provided for the residual transfer developer. When appropriate charges are not provided for the residual transfer developer, the developer cannot be collected by the developing device, and an image having a satisfactory quality cannot be obtained.
It is one objective of the present invention to provide an electrifier that uses charge particles to perform electrification satisfactorily, and an image forming apparatus therefor.
It is another objective of the present invention to provide an electrifier that applies appropriate electric charges to the developer on a charged rotary member, and an image forming apparatus therefor.
It is an additional objective of the present invention to provide an image forming apparatus that controls the electrical charges held by the developer. on a charged rotary member, so as to provide increased efficiency for the collection, by a developing device, of the developer on an image bearing member.
The other objectives and features of the invention will become more apparent during the course of the following detailed explanations of the preferred embodiments, given while referring to the accompanying drawings.